This invention relates to a method and structure for fabricating an improved static-pitot pressure transducer.
The ability to determine air from the measurement of the static (barometric) air pressure and the pitot air pressure, induced by the movement of a vehicle through air, has long been known from Bernoulli""s theorem for total air pressure in an incompressible flow, such as through air (below 250 mph) or any other incompressible fluid. Bernoulli states that the static pressure plus the dynamic pressure is equal to the total pressure. This can be expressed by the following equation:             P      S        +                  1        2            ⁢      ρ      ⁢              xe2x80x83            ⁢              v        2              =      P    T  
Where
PS=Static pressure
PT=Impact Pressure
xcfx81=density of the fluid
v=fluid velocity
For an ideal incompressible gas we can use the relation for density:   ρ  =                    P        S            ⁢      M        RT  
And obtain an equation for velocity:   v  =                              2          ⁢          RT                M            ⁢              (                              P            Δ                                P            S                          )            
Where
Pxcex94=PTxe2x88x92PS 
M=Molecular Mass
T=Absolute Temperature
R=The Universal Gas Constant
For higher speeds (between 250 and 750 mph) Bernoulli""s equation does not apply because the air becomes compressible. For this case another equation must be used:                     v        2            2        +                  h        ^            S        =            h      ^        T  
Where
v=velocity
h=enthalpy (the total internal energy plus pressure divided by density)
Using several simple relations for temperature and pressure in an ideal gas it is possible to obtain a relation for the velocity:   v  =                              2          ⁢          γ          ⁢                      xe2x80x83                    ⁢                      RT            s                                    M          ⁢                      (                          γ              -              1                        )                              ⁡              [                                            (                                                                    p                    Δ                                    +                                      p                    s                                                                    p                  s                                            )                                                      γ                -                1                            γ                                -          1                ]            
Where
xcex3=the ratio of specific heats CP/CV 
In either case, to obtain air speed v, one must accurately measure PS, Pxcex94 and T and then perform the required computations. It is, of course, clear that the accurate determination of v requires very accurate measurements of the static pressure PS and the differential pressure Pxcex94. The basic concept of obtaining air speed from pressure measurements is very old, and there are countless methods and structures for accomplishing the same. However, most previous structures are either too inaccurate, too large, too costly or too fragile and often all of the above. For example, a pitot tube air speed indicator consists of two elements, where one is a dynamic tube which points upstream and determines the dynamic pressure and the other is a static tube which points normal to the air stream and determines a static pressure at the same point. These tubes are connected to two sides of a manometer or an inclined gauge such as to obtain a reading of velocity pressure, which is the algebraic difference between the total pressure and the static pressure. In any event, such tubes have been used in aerospace applications and can also be used as a liquid flow-measuring device, but because of their tendency to clog, cannot be used with liquids which have suspended solid matter.
This is an example of a very old prior art device, which has many, many problems including being very large, fragile and so on.
It is therefore an object of the present invention to provide an improved method to produce a smaller, cheaper, more rugged, highly accurate static pitot pressure transducer, which is also capable of accurately measuring air speed.
The present invention employs two uniquely designed dielectrically isolated leadless piezoresistive semiconductor sensors on a specially designed Pyrex glass header using an inorganic ceramic glass to secure the diaphragm to the header and a glass metal frit to interconnect the two diaphragms to the requisite pins or terminals on the header. One of the sensors is designed to measure absolute pressure and, as such, has a sealed cavity, while the other sensor is designed to measure differential pressure and, as such, has an aperture which permits the pressure media to reach both sides of the sensor. The header itself has a through hole connected to a tube over which the differential sensor is affixed. The Pyrex glass is chosen so as to match the expansion coefficient of the sensor. The header is attached to a tubular member provided with a fitting such that static pressure can be applied to both sensors simultaneously. At the other end of the tubular member, there is provided another fitting attached to the tube in the header through which the total pressure can be applied to the differential sensor, thus permitting the measurement of the difference between the total pressure and the static pressure. Each of the sensors is fabricated using the same processing techniques and can be fabricated from the adjacent pieces of the same semiconductor wafer. The sensors are of the same thickness, but have different deflecting areas such that the differential sensor would have the greater stress upon it for the same pressure. The design of the sensors and the header leads to several unanticipated advantages. Since the expansion coefficient of the Pyrex matches that of the sensor so closely, not only is the induced thermal stress very low, but it does not vary with time resulting in an apparatus that is much more stable. In addition, since the sensors have the same thickness and are made by the same process, they match each other more closely allowing their individual variations to cancel each out.
On the total pressure end of the tubular structure there is also affixed a probe containing an RTD (a resistive temperature device) such that the temperature of the dynamic airflow may be measured. There is also provided various electronic circuits so that the air speed and other parameters may be accurately calculated based on the measurement of the differential and static pressures as determined by the above noted-equations.